1. Field of the Invention
The present invention relates to a method and an apparatus for analyzing impurities in gases, and particularly to a method and an apparatus for analyzing trace impurities contained in gases having high adsorptivity or reactivity such as oxygen, silane, diborane, phosphine, arsine or the like.
2. Description of the Prior Art
There have been conventionally carried out a method where a gas chromatograph provided with a column for separating impurities in sample gases and a mass spectrometer which is a detector carrying out analysis of the impurity components are combined such that the trace impurity components to be measured are separated in the gas chromatograph and then, the respective components are measured in the mass spectrometer. However, according to this method, when 10 ppb level of impurities contained in, for example, oxygen which is one of the gases having high adsorptivity or reactivity is analyzed, even though the sample gas is introduced several times, background impurities to be measured are not stabilized and at the same time peak height of the impurities is always varied for each introduction of the sample gas. Therefore, when impurities in oxygen are analyzed, it is necessary to carry out an operation of stabilizing background and peak of the impurities by introducing oxygen scores of times through an operating and sampling valve, every time before starting measurement. Furthermore, when columns are separately used according to the impurities to be measured, it is necessary to carry out the same operation for the respective columns which thereby requires much labor and time.
Furthermore, when an atmospheric pressure ionization spectrometer is used as a detector for the impurity component, since the impurities are measured with higher sensitivity, for example, from 1 ppb to sub ppb level, stability of the background or peak height of the impurities is much affected by the mass spectrometer which thereby requires a much longer time for obtaining stable measurement values.
More particularly, with reference to oxygen analysis, Japanese Laid Open Gazette Hei 9-236564 discloses a detection method where parts having adsorptivity or reactivity within an apparatus are made to be saturated with oxygen by introducing carrier gas containing a small amount of oxygen in advance and then, sample gas is introduced for detection of a small amount of oxygen therein. This is a method where oxygen as an impurity is detected by using only a gas chromatograph 10 and the ppm level of impurity oxygen is detected while several ppm of oxygen is always introduced into the carrier gas.
However, since a regular amount of oxygen is introduced into the carrier gas, in the case of an analyzing apparatus which combines a gas chromatograph with a mass spectrometer or an atmospheric pressure ionization spectrometer, a relatively high-concentration oxygen of several ppm is always present in the analyzing part of the mass spectrometer or the atmospheric pressure ionization spectrometer. Thus, a problem is that deterioration of the ion source of the mass spectrometer or the atmospheric pressure ionization spectrometer is expedited, and the charge of ions of impurities are transferred to the oxygen which is easily ionized to thereby make it difficult to detect impurities in the case of the atmospheric pressure ionization spectrometer, and therefore sensitivity is decreased.
Therefore, the object of the present invention is to provide a method and an apparatus for analyzing impurities in gases capable of shortening conditioning time, and which is capable of obtaining stable measurement values with superior reproducibility in a short time, even when the atmospheric pressure ionization spectrometer is used as a detector.
To accomplish the above-mentioned object, according to the present invention, there is provided a method for analyzing impurities in a sample gas in a spectrometer after separating said impurities from columns in a gas chromatograph, said method comprising the steps of: with said spectrometer operationally off, introducing a predetermined amount of said sample gas into the analyzing system plural times for a predetermined time interval and turning said spectrometer operationally on and starting analysis of said impurities.
Furthermore, according to the present invention, there is provided an apparatus for analyzing impurities in gas, comprising columns for separating said impurities in gas and a detector for analyzing said impurities separated from said columns, said apparatus comprising: sample gas introduction passages diverged in plural wherein said respective passages are respectively provided with said column and sampling valve for metering and supplying said sample gas into said column, and a switch valve for switching gas flowing out from any one of said columns into a passage for supplying said gas into said detector via analyzing passage, and switching gas flowing out from the other of said columns into a passage for exhausting said gas outside of the analyzing system via an exhaust passage wherein said switch valves are provided downstream of said columns.